(Frodo) Several changes in header parsing
The definition of header section is changed to exclude the long with the
offset of the Section Table Section. This allows easier integration of
Data-like file formats.
psiconv_parse_{texted,word,mbm}_file now do not parse the header section.
This will allow easier integration for objects-within-objects. They start
at the long with the offset of the Section Table Section.
psiconv_file_type now returns the read header section, and its length.